Electric heating unit



March 7, 1950 T. H. LENNOX 2,499,961

ELECTRIC HEATING UNIT Filed April 30. 1948 Fig.2.

Inventor- Thomas H. Lennox,

by @Mmam His Abbot-neg.

Patented Mar. 7,- 1950 ELECTRIC HEATING UNIT Thomas H. Lennox, Plttsfleld, Mass., assignor to General Electric Company, a. corporation of New York Application April 30, 1948, Serial No. 24,208

2 Claims. (Cl. 201-67) the provision of an improved method of making an electric heater of this character.

More specifically, my sheath wire units used in flat or platen heating and oven heating in each of which it is desired to maintain a uniform temperature over a flat surface. In these types of heaters, the units are disposed parallel to each other in a plane parallel to the workpiece.

My invention provides such a planar heat source at a much reduced cost. It was found that if uniform unit resistance were used over the whole len th of the heat ng units, more heat escaped into the air from the areas nearer the ends of the heating units than the areas nearer the center of the units causing uneven heating of the workpiece area.

The remedy sou ht to be applied for this situation was coils with tapered. 01' graduated, heating characteristics that is. cois that provide more heating pe unit len th at each end than in the center. The method first tried employed a reinvention relates to sistance coil of uniform diameter. Before crimping or welding to the coil terminal arbors, a center section was stretched to fewer turns per un t len th. The coil was then centered in a sheath and insulation was vib atin ly packed around it. The entire unit was subjected to a swaging operation in which the cross-section area was reduced and the unit len thened.

This method has been found to have some disadvanta es since the prestretched section of coil is less resilient than the unstretched section and after the lengthening of the whole coil caused by swaging the unit. the resistance unit len th of the entire coil had a tendency to become uniform, instead of having the desired taper.

In carrying out my invention, I utilize helical resistance coils of diffe ent diameters for providing different rates of heat generation. In one form of my invention I use a resistance coil with its center section wound to a smaller diameter than that of its ends. It has been found that when the center section of such coil is prestretched, the resulting unit has the desired tapered heat producing characteristics.

Reference to the accompanying drawing will add to a more complete understanding of my invention, in which Fig. 1 is a sectional view of the complete tapered heat Calrod unit; Fig. 2 is a view of the coil partially wound upon its arbor; Fig. 3 is a view of the arbor alone.

Referring to the drawing of one embodiment of my invention, an arbor in Fig. 2- comprises a rod I upon which two sleeves 2 and 3 are slidably mounted and to which they are removably secured by the set screws 4 and 5 in the flanges of the sleeves, leaving the rod i uncovered by the sleeves in the middle section. The coil wire of high resistance material having considerable. resiliency such as nickel, chromium alloy is wound in a helix continuously with a uniform spacing of the turns, as shown, from one sleeve flange to the other, the outside diameters of the sleeves and the rod becoming the inside diameters of the coil. After winding the sleeves and shaft are withdrawn, leaving the coil with a middle section diameter smaller than the equal end diameters. The elongation per unit length produced by the swaging of a unit comprising a coil wound with different diameters is not uniform since the coil length of smaller diameters is less resilient than the remainder, therefore, this middle section is stretched to nearly the desired length before the swaging, rolling or drawing, operation to reduce the diameter in order to compact the powdered insulation further.

Terminal rods 6 and l of the same material as that of the coil are firmly attached to the coil at their ends 8 and 9 of larger diameter and the coil is mounted concentrically in the tubular metallic sheath l0 under tension enough to maintain its concentric position, and with terminals 6 and I firmly attached to two sheath end caps. Ceramic spacers ll, l2, I3 and M are provided at suitable intervals to maintain the coil concentric with the sheath. The sheath is held upright and continuously vibrated while a powdered insulation material, like magnesium oxide, is poured into the tube through apertures provided in one end cap for the purpose. Magnesium oxide is used as insulation because of its high heat conductivity when its powdered form is compressed. When filled the end caps are replaced by washers to which the terminal rods are securely fastened. These washers keep the packed insulation within the tube during the reduction of cross-section operation which follows.

This cross-section reduction operation increases the length 01' the unit about 20% with an accompanying decrease in diameter, packing the insulation firmly within the sheath. The sheath is then cut to the desired length leaving the terminal rods at each end projecting for attachment in an electric circuit. This assembly of the resistance unit and. sheath and subsequent swaging is described more in detail in The stretched length of-the reduced diameter center section of coil is chosen so that after.

reduction its length will be substantially equal to that of each end section. The relative diam eters in a typical heating unit are such that'the resistance of each end section is substantially equal to one and one-half the resistance of the middle section. Since the rate of heat generation in a coil is proportionate to the coil resistance, in such a typical heating unit, the heat is generated substantially one-quarter by the middle section and three-eighths by-each end section. It is to be understood that this is but one form or my invention and that these relative values may be changed at will. 1

Fig. 3 illustrates an arbor for use in another form of my invention in which the end sections of the coil are tapered toward each end from the middle section, with parts corresponding to the parts of the arbor in Fig. 2 described above and numbered accordingly.

While certain specific embodiments have been shown and described, it will, of course, be understood that various modifications may be made without departing from the invention. The appended claims are, therefore, intended to cover any such modifications within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent oi the United States is:

1. A metallic sheathed heating unit having greater heat generation in a section at each end than in the remaining central section, comprising a tubular metallic sheath, powdered heat refractory insulating material compacted to a hard dense mass in said sheath, and a continuous helical coil of resistance wire in said sheath extending lengthwise thereof and em bedded in said insulating material having [its turns spaced apart substantially uniformly throughout-its length and being provided with at least three sections each having turns of uniform diameter, there being a section at each end having a substantially greater diameter than the remaining central section to provide a greater length of said resistance wire in said end sections per unit length oi said sheath for the generation of a greater amount of heat per unit length in said end sections as compared with said central section.

2. A metallic sheathed heating unit having greater heat generation in a section at each end than in the remaining central section, comprising a tubular metallic sheath, powdered heat refractory insulating material compacted to ahard dense massin said sheath, a continuous helical coil of resistance wire in said sheath extending lengthwise thereof and embedded in said insulating material having its turns spaced apart substantially uniformly throughout its length and being provided with at least three sections each having turns of uniform diameter, there being a section at each end having a substantially greater diameter than the remaining central section to provide a greater length of said resistance wire in said end sections per unit length of said sheath for the generation of a greater amount of heat per unit length in said end sections as compared with said central section, an electric terminal rod projecting fromeach end of said sheath having its inner end embedded in said insulating material, and en-' larged portions on the inner ends of said terminal rods secured respectively to the ends of said eoii.

' THOMAS H. LENNOX.

The following references are of record in the file of this patent:

UNI'IED STATES PATENTS Number Name Date 563,032 Hadaway, Jr. June 30, 1896 essence Lightfoot Nov. 16, 1920 1,755,444 Von Brockdorii Apr. 29, 1930 1,901,892 Ejorkman et a1. Mar. 21, 1933 1,911,063 Daiy May 23, 1933 

